The first says "I'm getting old" and lets his body do what poorly maintained bodies do.
He loses a step. Then the stairs are the problem. Then getting up off the floor.
The second says the exact same sentence, "I'm getting old," and trains harder because he knows what's coming.
Same words. Same age. Completely different Tuesday in ten years.
One is preserving a decline. The other is building capacity.
I spent twenty years in emergency and flight medicine watching which one shows up on a stretcher.
It is rarely the one who was training.